Great Lakes Resource Sheds
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Resource sheds delineate areal distribution of materials prior to its passing through some point of interest during a given time period.
Consider a location in the Great Lakes with material passing through it (e.g., water, pollutant, sediment). Where did the material come from and how long did it take to reach the location?
What is a Resource Shed?
Resource sheds are geographic areas contributing resources to a point location, population, or organism over a specified time period. For example, a one-month resource shed for a point location would represent the area supplying resources to the location over the period of a month. The scientific question dictates the relevant time period. We can specify resource sheds for specific resources or generally for those resources influenced by common physical forcing variables, such as currents. We can also specify resource sheds for populations, organisms, individuals, or life-stages. The term is analogous to, or rather a generalization of, “watershed” or “airshed”. Indeed, employing “shed” utilizes its meaning "to impart", as in "an area that imparts resources".
The concept of the resource shed is not new, and was derived from Ecosystem Trophic Modules (Cousins 1990, 1996), formalized by Holt (1996) and Polis (1997), and promoted by Power and Rainey (2000).
It is easiest to explain how resource sheds can be useful by looking at a few examples of resource shed maps. The definition of resource shed refers to the spatial extent of a contributing area, but does not address the differences between parts of a contributing area. Some parts of an area may supply more material to our location of interest than other parts. See for example the 1-day resource shed distributions in Figure 1a.
Figure 1a: Example of the Maumee watershed as a resource shed for water leaving the watershed on June 25, 2008 that started movement in the last 24 hours. The subset of the watershed that is acting as the resource shed for this time period is outlined in yellow.
A much different resource shed map shows the material density distribution for water leaving the watershed on June 25th that started movement in the past 4 days. In this case, the entire watershed is the resource shed. It is clear from Fig 1b that there was a storm event impacting this watershed on or around June 21st.
Figure 1b: Maumee 4-day resource shed distribution for water leaving the watershed on June 25, 2008 that started movement in the last 4 days.
Estimating Resource Sheds
In a lake, both resource sheds and resource shed distributions may be computed with a lake circulation model in which particles (tracers) are defined at a given location and the model is run backward in time from a specified time. The locations of the particles at earlier times define the extent of the resource shed. The density of the particle distribution over the resource shed defines the fraction of material contributed (resource shed distributions); see Figure 1. However, in a watershed, resource sheds and their distributions are computed from spatially distributed watershed hydrology models, which generally are not particle models; instead, material placed anywhere in the watershed will appear at the watershed outlet (mouth) over a period of time. We calculate the material appearing at the mouth at any time, contributed by a specific area. Since most distributed watershed hydrology models cannot be run backward in time, we systematically consider every “cell” (say 1 km 2 ) of a watershed surface by tracing the material departing the cell over time interval i and computing the amount arriving at the mouth in time interval j (definition one). This requires modeling all cells, but tracing contributions from only one at a time. Since it involves no more computation, we simulate material movement from a cell during time intervals i,......,j to the mouth in time interval j (definition two). We can combine results of several sets of simulations to determine the source distribution of departing material in the watershed going through the mouth in any specified time period (definition three). We can also calculate relative fractions as well as absolute amounts by dividing by the total amount of material involved; they will be useful as source probability density estimates.
Figure 2: Example estimating resource shed distribution by tracking particles backwards
Examples of Resource Sheds
Watershed Resource Shed
Note that the classical “travel-time” isochronal map for a watershed (Linsley et al., 1982) is an example of our definitions. It is built, by assuming steady uniform rainfall, to estimate a watershed’s time-area histogram, which is then used further to estimate the unit hydrograph for a watershed. For example, consider the mouth of a watershed at time 0 with constant outflow resulting from sustained spatially and temporally uniform precipitation over the entire watershed. If we determine the travel times (d) from all locations in the watershed to the mouth and plot them, we have the classical hydrological travel time isochronal map shown on the left side of Figure 3 for an arbitrary watershed. The resource shed for water departing during the ith time interval (i = -1, -2,...,) and arriving at the watershed mouth during the 0th time interval is that area with travel times within [(i-1)d, jd); the shaded area in Figure 2 a resource shed for water leaving during the 3rd time interval and arriving during the 0th time interval. By identifying similar resource sheds for other time intervals from the isochronal map, we can build the “time-area” histogram of classical hydrology, shown on the right side of Figure 3. The shaded bar in the time-area histogram corresponds to the shaded resource shed in the isochronal map. Alternatively, the resource shed for water starting movement during time intervals i—0 and leaving the watershed at time 0 are calculated by summing areas between the isochrones in Figure 3.
Figure 3: Isochronal time travel map and resultant time-area histogram
Ecological Resource Shed
Zebra mussels provide a useful example to define resources sheds. Imagine a mussel in stagnant water. As the mussel feeds it depletes the water immediately surrounding itself. As time progresses (T), the area depleted will grow (Fig. 4a). This area is thus supplying resources to the mussel, and is the resource shed. Within this context, “resources” can mean anything that can be transported by currents, e.g. suspended nutrients, particulate organic matter, sediments, phytoplankton, propagules, or pollutants. Because water circulates, currents transport resources to new locations. Thus in nature mussels actually consume resources that are continuously coming from somewhere else (Fig. 4b). That “somewhere else” will be defined by circulation patterns. Hence we can define resource sheds in coastal ecosystems as the areas expanding “up-current” that supply resources to “target” locations of interest over specified time periods (Fig.4c). Extend those time periods back far enough, and the resource shed will eventually hit the coast and intersect with tributaries (Fig. 4d). Hence resource sheds in coastal ecosystems are a function of circulation patterns initially, and watershed inputs ultimately.
Lake Resource Shed
Particle tracking provides a convenient and useful means of visualizing flow patterns. Please refer to Figure 1 below to see an illustrated example of how the particle tracking model works in delineating resource sheds. Figure 5a shows the 1-day, 1-week, and 1-month resource sheds for an example Lake Erie site. Figure 5b shows a density plot of the 1-month resource shed, illustrated the relative importance of areas in the resource shed. The shapes are the result of the river plume interaction.
Figure 5a: 1-day, 1-week, and 1-month resource sheds
Figure 5b: Density plot of the 1-month resource shed, illustrated the relative importance of areas in the resource shed.
Cousins, S.H., 1996, Food webs: From the Lindeman paradigm to a taxonomic general theory. In: Food Webs. Integration of Patterns and Dynamics (eds G.A. Polis and K.O. Winemiller), pp. 243-251. Chapman and Hill, New York.
Holt, R.D., 1996, Food webs in space: an island biogeographic perspective. In: Food Webs (eds. G.A. Polis and K.O. Winemiller) Chapman and Hall, New York.
Linsley, R. K., Jr., M. A. Kohler, and J. L. H. Paulhus, 1982. Hydrology for Engineers, Third Edition, McGraw-Hill, New York, New York, 508 pp.
Polis, G. A., W. A. Anderson, and R. D. Holt, 1997, Toward an integration of landscape and food web ecology, Annual Review of Ecology and Systematics, 28:289-316.
Power, M. E., and W. E. Rainey, 2000. Food webs and resource sheds: towards spatially delimiting trophic interactions, pp.291-314 in Hutchins, M. J., John, E. A., and Stewart A. J. A. (eds), The Ecological Consequences of Environmental Heterogeneity, Blackwell Science.